A common method of packaging products involves forming a single sheet of film into a pouch having an open end, filling the pouch with the product through the open end, and then heat sealing the open end. This method is especially popular in packaging food products, particularly potato chips and other snack foods and cereals. The packaging film normally has at least two layers: a base layer which provides the desired barrier properties and a heatseal layer. High density polyethylene (HDPE) is particularly suitable for forming the base layer because of its excellent moisture barrier properties. However, HDPE has a relatively low melting point (about 270.degree. to 275.degree. F.), which limits the materials that may be used as the heatseal layer.
Normally, the heatseal is formed by fusing opposing sections of the heatseal layer together by the application of heat and pressure. The standard procedure in the industry is to apply a pressure of about 20 psi for one second. The amount of heat applied is that which is sufficient to fuse the heatseal layers together without affecting the base layer. Therefore, the fusion temperature of the heatseal layer must be less than the heat distortion temperature of the base layer, which for HDPE is about 250.degree. F.
The strength of the heatseal must be great enough so that the package is able to withstand normal handling without opening before actual use. It has been found that this heatseal strength must be greater than about one pound per inch width (PIW), and preferably is greater than about 1.5 PIW. On the other hand, the strength of the heatseal must not be so great that the package is difficult to open by the consumer. To be easily opened, the heatseal should have a heatseal strength of less than about 5 PIW, preferably less than 3 PIW.
Opening of the heatseal involves rupture of the heatseal layer followed by delamination of the heatseal layer from the base layer, as shown in U.S. Pat. No. 3,655,503. The force required to rupture the heatseal layer is referred to as the "peak strength"; and the force required to delaminate the heatseal layer from the base layer is referred to as the "peel strength". The peak strength is usually greater than the peel strength. The peak strength depends on the tensile strength of the material forming the heatseal layer and the thickness of the heatseal layer. Consequently, since the peak strength is normally greater than the peel strength, and therefore usually determines the heatseal strength, one might attempt to form an easily openable package (i.e. one having a heatseal strength of less than 5 PIW) by employing an especially thin heatseal layer. However, it has been found that in order to form a satisfactory heatseal, the heatseal layer must be at least 0.25 mil thick because in actual use the surface of the heatseal layer becomes contaminated by particles (e.g. cereal dust) picked up from the product when it is packaged. Thus, the heatseal layer must be at least about 0.25 mil thick in order to flow around the particles and thereby prevent them from interfering with formation of the heatseal. On the other hand, the heatseal layer must be thin enough (generally less than about 0.75 mil thick) so that after the heatsealing operation it cools quickly enough so that the seal sets before the weight of the product acting against the interior walls of the package has a chance to force open the seal. Similarly, the heatseal layer must have sufficient tack strength at the fusion temperature to hold the seal closed until it has set.
Another requirement of the material forming the heatseal layer is that it must be coextrudable with the base layer to form the packaging film.
Consequently, in a packaging film comprising a heatseal layer laminated to a base layer of HDPE, the heatseal layer must meet the following critical specifications: (1) it must have a fusion temperature of less than about 240.degree. F., (2) at a thickness between 0.25 mil and 0.75 mil, it must form a heatseal bond greater than about 1 lb/inch width and less than about 5 lbs/inch width when a pressure of about 20 psi is applied for one second at a temperature above the fusion temperature, (3) it must have sufficient hot tack strength at the fusion temperature to prevent the seal from opening as it cools after the heatsealing operation, and (4) it must be coextrudable with the base layer. Because of the excellent moisture barrier properties of HDPE, there has long been a need for a packaging film comprising a base layer of HDPE laminated to a heatseal layer meeting these specifications, but as far as is known, no such film was ever developed prior to this invention.